1. Field of the Invention
The present invention relates to a method of grasping the distribution state of a compound in soil. More specifically, the present invention relates to a method of detecting the high-concentration region direction of a compound in soil using a frequency-converting element or the like. In this specification, the term “soil” includes all components composed of a solid, a liquid and a gas, for forming the earth and groundwater.
2. Related Background Art
There are prior arts such as “Search for chemical substance source by means of submerged compass system” and “Research on compass for searching chemical substance source in water imitating crayfish” (see “Data of the research conference on a chemical sensor by the Institute of Electrical Engineers” VOL. CHS-03, NO. 56-86; PAGE. 75-80; (20031128-20031129), Hiroshi Sakata, Seigo Ito, Hiroshi Ishida, Toyosaka Moriizumi, (Tokyo Institute of Technology), “Convention record of the Institute of Electronics, Information and Communication Engineers” VOL. 2004, Electronics 2; PAGE. 151; (20040308), Hiroshi Sakata, Hiroshi Ishida, Toyosaka Moriizumi, (Tokyo Institute of Technology) (hereinafter, referred to as “Document 1” and “Document 2”, respectively)).
Hereinafter, a conventional method of detecting the direction of the position of a pollutant will be described with reference to Documents 1 and 2.
Document 1 discloses “A compass for searching an odor source in water capable of detecting a chemical substance by actively sucking water around the substance was developed. A slit was arranged immediately before a sensor electrode arranged at the inlet of a circular pipe to enable a sucked chemical substance to be surely captured by the sensor electrode. In addition, it has been confirmed that the range of directions to be sucked by each slit can be controlled by changing the number of slits and the width of each slit. A submerged compass having 4 sensor electrodes was produced. As a result, observing which sensor responds allows one to judge whether a substance source is present in the range of the forward direction ±20°, or left or right region except for the range. The compass system was attached to a linear actuator to perform an experiment for searching a chemical substance source in still water in a water tank. As a result, the search was successful at a high probability.”
Document 2 discloses “A crayfish can attract not only an odor isotropically from its surrounding but also a flow intensively from one of its right and left sides by using its fan-like appendages called gnathopods. The present document realized a function of sucking from only a specific direction, by imitating the gnathopods of a crayfish to separately use right and left gnathopod structures. The use of a gnathopod structure makes it possible to realize a system for searching the presence or absence of a source by efficiently collecting signals from various places around the system.”
Document 1 describes a system for detecting a chemical substance by actively sucking the substance through a slit. When the system is applied to the search for the direction of a volatile chlorinated organic compound (VOC) in soil, differences in the rate of a sucking stream and the rate of a sucking wind occur owing to differences in granularity and porosity in the soil other than the slit, to thereby destabilize a concentration signal. In addition, in such system as described in Document 2, suction causes turbulence to make a concentration signal more unstable than that in the present invention. As described above, a suction system destabilizes a concentration signal and deteriorates a detection lower limit. Therefore, such system is not suitable for detecting a VOC at an extremely low concentration and for detecting a direction of interest. In addition, the system described in Document 2 does not express a suction function in a condition like soil where the amount of dirt is larger than that of water because the wing of a gnathopod structure does not move in such condition, although the system can express the suction function in water.
Investigation into the distribution of pollution by a chlorinated organic compound such as trichloroethylene that pollutes soil has been conducted by detecting and measuring a volatile chlorinated organic compound by means of a detecting tube in a boring hole at a certain interval or in a shallow excavated hole. However, tracking down a region where a high concentration of chlorinated organic compound is present is extremely difficult. A chlorinated organic compound that has polluted soil is dissolved into groundwater and soil water to diffuse, and is vaporized to diffuse and move in the soil. The movement causes the compound to distribute in the soil while forming a certain concentration gradient in the soil.
A large number of methods each involving analyzing a gas sample collected from soil by means of an analyzer such as a gas chromatograph have been adopted for detecting a chlorinated organic compound such as trichloroethylene. In addition, Japanese Patent Application Laid-Open No. 2004-108913 discloses a gas measurement method including: subjecting a gas to be measured to oxidation and reduction decomposition in a reaction tube to transform the gas to be measured into a decomposition product; and detecting the gas by means of a chemical reaction between the decomposition product and a quartz resonator electrode material or adsorption property between them. However, the method is not intended for detecting a distribution. In addition, Japanese Patent No. 3443632 discloses, as a technique for direct detection, a method of measuring a gaseous chlorinated organic compound by means of a quartz resonator having an electrode to which lipid is applied. However, the method is intended for detecting the concentration of a gaseous chlorinated organic compound, and it is not possible to detect the distribution state of a chlorinated organic compound in soil in soil water, ground water, or the like.
Japanese Patent No. 2759683 discloses the constitution of a quartz resonator for detecting a compound in water by means of a quartz vibrating element. Here, a material for adsorbing or absorbing the compound in water is applied to one surface of one electrode of the quartz resonator, and one surface of the other electrode of the quartz resonator is coated with an electrically insulating jacket so that the electrode is out of contact with water and is in contact with the air.
The above shape has a large jacket with which the quartz resonator is coated, and involves the difficulty in grasping a change in distribution of the concentration of a VOC.
Japanese Patent No. 3292866 discloses a method of detecting the flow of a gas or an odor as a gas. The method involves: arranging a large number of sensors for detecting a gas on a plane with a spatial spread; and grasping the flow of a gas or odor upon response by each sensor element. However, if it is assumed that the method is used for measurement in soil, the sensors must be installed in a limited well or excavated hole. Therefore, it is difficult to detect the flow and direction of a gas when it is difficult to arrange the sensors widely and two-dimensionally or three-dimensionally.